•One-step anodization (OSA) and two-step anodization (TSA) of Ti are compared.•Constant current anodizations were designed to control charge transfer.•Essential distinction between the OSA and the TSA was analyzed for the first time.•The distinction was clarified by oxygen bubble mould and plastic flow model.•The concaves contribute to the growth of highly-ordered ATNTs during TSA.One-step anodization (OSA) and two-step anodization (TSA) have been widely used to fabricate anodic TiO2 nanotubes (ATNTs). It has been reported that TSA can fabricated more ordered nanotubes, as compared to OSA. However, the essential distinction between OSA and TSA of Ti has never been clarified. Here, constant current anodizations were designed to study the effects of TSA and OSA on the morphology of ATNTs. The significant differences between nanotubular architecture (length, diameter, porosity) of ATNTs fabricated by OSA and TSA were observed and analyzed for the first time. For TSA, the relationship between the nanotube length (y) and anodizing current (x) corresponds to the liner equation y = 0.259x-2.3, and the relationship between nanotube outside diameter (Y) and anodizing current (x) corresponds to the linear equation Y = 3.352x+65.73. However, no similar linear relationships were found for OSA. The essential distinction between OSA and TSA was clarified by oxygen bubble mould combined with plastic flow model.Download high-res image (116KB)Download full-size image

A series of novel nontoxic near-infrared (NIR) reflective pigments based on Al-doped BiFeO3 coated mica-titania were synthesized by precipitation combined with sol-gel method. The pigments of the formula BiFe1−xAlxO3/mica-titania (x = 0, 0.1, 0.2, 0.3, 0.4) were characterized by XRD, FE-SEM, TG-DTA, UV–vis–NIR spectrophotometer and CIE L* a* b* color scales. The results illustrate that the BiFeO3 nanoparticles are coated on the surface of mica-titania uniformly, and the doped BiFe1−xAlxO3/mica-titania is similar to BiFeO3/mica-titania composite in morphology. Furthermore, the absorption edge of composite pigments shift to shorter wavelength (533–495 nm) can be attributed to O2p-Fe3d charge transfer transitions and change the color of the pigments from brown to orange. Additionally, the NIR solar reflectance of the powdered pigments and pigmented coatings were measured. The results reveal that with the increase of progressive doping of Al3+ for Fe3+, the NIR solar reflectance of the pigments increase gradually and exhibit higher NIR solar reflectance (R* ≥ 47.8%) than the conventional pigment of similar color. Moreover, we also evaluated the thermal and chemical stability of the pigments. In conclusion, the pigments have the potential to be applied as “cool pigments”.

•A novel near-infrared reflecting inorganic pigments Fe/N co-doped MgTiO3 have been prepared by the Sol-Gel method.•Fe/N co-doped MgTiO3 displays wide range of colors from white to red-brown.•The pigments possess high NIR solar reflectance (>53.51%).•Indoor air temperature difference of boxes in the simulation test reaches 2.0–3.0 °C.•The interior surface temperature difference of the two pigmented coatings is above 2.4 °C.To obtain cool materials as inorganic and nontoxic pigments with high near-infrared reflectance, nano-powder of Fe/N co-doped MgTiO3 (FNM), were synthesized by the sol-gel method. The substitution of Fe/N changed the color from white to dark-red due to the point defect equilibrium theory, while the band gap decreased from 2.11 ev to 1.89 ev. Ammonolysis reaction can promote the particle dispersion and increase the particle size. Furthermore, the FNM pigments exhibit high NIR reflectance (>65%) and the NIR solar reflectance (>53.51%) in the range of 700–2500 nm. After the prepared pigments (N-doped Mg1−xFexTiO3+δ, x=0.3) and conventional brown pigments used as building roof materials, the temperature difference of coatings could reach 2.4 °C (the inner surface temperature of substrate plates) and 2.0 °C (the interior temperature of the two boxes), respectively. Moreover, the surface temperature distribution of the coatings was recorded by infrared images and the difference (ΔT) varied at 0–12 °C. According to the maximum value of coatings (x=0.3), energy savings would reduce annual emissions of 1379.35 t carbon dioxide, 41.65 t nitrogen oxides and 34.30 t sulfur oxides. Owing to the wide range of color and good chemical stability, which meets reflectance of solar radiation energy and cooling energy needs, these FNM materials are considered as novel color pigments.Download high-res image (243KB)Download full-size image

•A novel inorganic pigments LaFe1−xAlxO3 have been prepared with a Sol–Gel method.•Replacing Al3+ for Fe3+ in LaFeO3 displays wide range of colors from brown to yellow.•The pigments possess high NIR solar reflectance (>52% ).•Indoor air temperature difference of boxes in the simulation test reaches 5.4 °C.Novel, brown and yellow inorganic pigments LaFe1−xAlxO3 (x=0, 0.1, 0.3, 0.5, 0.7) with high near-infrared (NIR) reflectance have been prepared by a Sol–Gel method. Replacing Al3+ for Fe3+ in LaFeO3 changed the color from brown to yellow and the band gap increased from 2.46 ev to 2.77 eV due to the quantum confinement effect arising from the small size regime. Moreover, the synthesized pigments and the coatings colored with them possess high NIR solar reflectance in the range of 700–2500 nm. The temperature difference between the pigments we prepared and standard pigments was evaluated in simulating their usage as building roof materials. The thermal and chemical stability of the pigments were also studied.

•A novel inorganic pigments Zn1−xMgxFe2O4 have been synthesized with a Sol–Gel method.•Replacing Mg2+ for Zn2+ in ZnFe2O4 changed the color from brown to dark brown.•The pigments possess high NIR solar reflectance (>51%).•The surface temperature difference of the two pigmented coatings is above 7 °C.•Indoor air temperature difference of boxes in the simulation test reaches more than 6 °C.A new class of environmentally benign inorganic pigments with general formula Zn1−xMgxFe2O4 (x = 0, 0.1, 0.2, 0.3, 0.4) were successfully prepared by the Sol–Gel method. The developed nano-crystalline powders were characterized by TG-DSC, XRD, FESEM, UV–vis–NIR diffuse reflectance spectra, FLIR, and CIE-L∗a∗b∗ 1976 color scales. The results demonstrate that the single-phase Zn1−xMgxFe2O4 is synthesized at an optimum temperature of 800 °C. The resulting calcined powders are nano-sized particles with a well-developed cubic spinel structure. The doping of magnesium in zinc ferrite changes the color from brick red to dark brown and the band gap shifts from 2.23 to 2.18 eV. Mg doped pigments possess high near-infrared solar reflectance (>51%). In the simulated experiment, the analyses of thermal performance reveal that the surface temperature and the interior temperature of the devices covered with the coating we prepared are lower than those with the conventional coating, and the coating colored with the pigment we prepared absorbs less NIR portion and decreases the heat flux transferred into indoor space, which can decrease the need of cooling and increase thermal comfort in the summer. Therefore, these Zn1−xMgxFe2O4 powders have great potential in serving as cool pigments for building coatings.

•Na2V6O16·xH2O nanoribbon was utilized as an infrared reflective pigment for the first time.•Nanoribbons possess high NIR solar reflectance of 59.1% compared with nanoparticles.•The reduction in inner surface temperature of reflective panel in the device is 4.5 °C.•The estimated energy saving is 27.2 W m−2 (10.8%).•The Na2V6O16·xH2O nanoribbon can well serve as a cool brilliant brown pigment.The ultra-thin and super-long Na2V6O16·xH2O nanoribbon was for the first time utilized as a cool color pigment with high near-infrared reflectance and thermal performance. The nanoribbon appears brilliant brown with red and yellow hues and the spectral absorption edge is below 575 nm. The nanoribbon has high spectral reflectance (62.4–76.2%) in 700–1000 nm wavelength region and near-infrared solar reflectance (59.1%) compared with its nanoparticle counterpart and conventional pigment of the similar color due to its unique morphology. In the experimental test, the coating colored with the nanoribbon reduced inner surface temperature of reflective panel by 4.5 °C and heat flux by 27.2 W m−2 (10.8%). Na2V6O16·xH2O nanoribbon pigment can effectively reduce solar heat gain and cooling energy demand.

•We have prepared a novel yellow pigments Y2Ce2−xFexO7+δ (x ranges from 0.00 to 0.15) with a Sol–Gel method.•The doping of Fe3+ changes band gaps of pigments from 3.28 eV to 3.15 eV.•The pigments possess very high NIR reflectance (above 80%).•The NIR reflectance of the pigment coatings reach 73.8%, and the temperature difference of coating and general pigment coating reaches to 3.3 °C.•These Y2Ce2−xFexO7 + δ pigments could be well serve as cool colorants.In this work, we have synthesized a new kind of inorganic yellow pigments Y2Ce2−xFexO7 + δ (x = 0.00, 0.05, 0.10, 0.15) with high near-infrared (NIR) reflectance, using the Sol–Gel method. The substitution of Fe3+ for Ce4+ in Y2Ce2O7 decreased the band gap of pigments from 3.28 eV to 3.15 eV attributed to O2p to Fe3d charge transfer transitions, which resulted in the color of powder pigments samples changing from ivory white to light yellow. The pigments possess high NIR reflectance (>80.6%) in the range of 700–2500 nm, and the coatings colored with pigments also possess high values (>65.3%) of the solar reflectance too. We have evaluated the indoor air temperature difference of pigment coatings used as building roof materials. In conclusion, these pigments could well serve as cool colorants.Graphical abstractThe doping of Fe3+ for Ce4+ in Y2Ce2O7 samples decreases the value of the NIR reflectance, but all of the pigment coatings still possess very high NIR reflectance (>63%) calculated with ASTM G159-98.